1DsIII and SDXC (apparent success)
I bought myself a 64GB micro SDXC (Sandisk Ulrta Mobile Class 10 / UHS-1) primarily for my tablet but thought I would give it a spin in my 1DsIII. As I expected the camera doesn't know about exFAT and the built-in formatter can't cope.
However, formatting it as FAT32 using http://www.ridgecrop.demon.co.uk/index.htm?fat32format.htm makes the card visible to the camera and the Format menu option can see the card but refuses to format and if you try you then need to power-cycle the camera to see the card again after the error. FYI The Format option reported 59.4GB size, matching the Windows reported size.
The camera showed 1,999 shots available in my normal RAW+Small (Q2) JPEG but dropped to a real number around 1,845 with RAW+Large (Q8) ISO200. I took about 70 shots before getting bored to make sure the files were reliably written and visible on my laptop in the SD slot and they were but I have not got anywhere filling the card up or passing anything like a potential 32GB barrier.
So, even if I bought it for movies and music, I am going to give it a try in the camera and carry a couple of my usual 32GB Lexar microSDHC cards "just in case" while having a 128GB Delkin, that I also just bought, in the CF slot which is going to have all the same shots on anyway again "just in case".